1. Field of the Invention
The present invention relates to a microlithography objective and, more particularly, to a microlithography projection objective for short wavelengths, preferably ≦93 nm, a projection exposure system that includes such a microlithography projection objective, and a chip manufacturing process that employs such a projection exposure system.
Lithography with wavelengths<193 nm, particularly EUV lithography with λ=11 nm or λ=13 nm is discussed as a possible technique for imaging of structures<130 nm, most preferably <100 nm. The resolution of a lithographic system is described by the following equation:RES=k1·λ/NA,wherein k1 denotes a specific parameter of the lithography process, λ denotes the wavelength of the incident light and NA is the numerical aperture of the system on the image side.
Reflective systems with multilayers are used as optical components for imaging systems in the EUV range. Preferably Mo/Be systems are used as multilayer systems at λ=11 nm and Mo/Si systems are used for λ=13 nm systems.
If a numerical aperture of 0.2 is used as a basis for calculation, then the imaging of 50-nm structures with 13-nm radiation requires a relatively simple process with k1=0.77. With k1=0.64, imaging of 35-nm structures is possible with 11-nm radiation.
Since the reflectivity of the multilayers used lies only in the range of approximately 70%, it is of importance to use as few optical components as possible in an EUV projection objective, in order to achieve a sufficient light intensity.
Systems with six mirrors have proven particularly preferred for NA=0.20 with a view toward a high light intensity and sufficient possibilities for correction of imaging errors.
2. Description of the Prior Art
6-Mirror systems for microlithography have been made known from the publications U.S. Pat. No. 5,153,898; EP-A-0 252,734; EP-A-0 947,882; U.S. Pat. No. 5,686,728; EP 0 779,528; U.S. Pat. No. 5,815,310; WO 99/57606; and U.S. Pat. No. 6,033,079.
The projection lithography system according to U.S. Pat. No. 5,686,728 shows a projection objective with six mirrors, wherein each of the reflective mirror surfaces is formed aspherically. The mirrors are arranged along a common optical axis such that an obscuration-free light path is obtained.
Since the projection objective known from U.S. Pat. No. 5,686,728 is used only for UV light with a wavelength of 100-300 nm, the mirrors of this projection objective have a very high aspheric departure of approximately ±50 μm as well as a very large angle of incidence of approximately 38° C. Also, after stopping down the aperture to NA=0.2, an aspheric departure of 25 μm from peak to peak remains here along with a barely reduced angle of incidence. Such aspheric departures and angles of incidence are not practical in the EUV range due to the high requirements for surface quality and reflectivity of the mirror.
Another disadvantage of the objective that was made known from U.S. Pat. No. 5,686,728 does not permit a use in the wavelength range of λ<100 nm, particularly at wavelengths of 11 and 13 nm, is the very short distance between the wafer and the mirror arranged next to the wafer. In the case of the distances between the wafer and the mirror next to the wafer, known from U.S. Pat. No. 5,686,728, the mirrors can only be made very thin. Due to the extreme layer stresses in multilayer systems for the discussed wavelengths of 11 and 13 nm, such mirrors are very unstable.
A projection objective with six mirrors for application in EUV lithography, particularly also for wavelength of 13 nm and 11 nm, has been made known from EP-A-0 779,528.
This projection objective also has the disadvantage that at least two of the total of six mirrors have very high aspheric departures of 26 or 18.5 μm. However, even in the arrangement known from EP-A-0 779,528, the optical free working distance between the mirror next to the wafer and the wafer itself is so small that the mirror next to the wafer has instabilities or a mechanical free working distance that is negative.
A 6-mirror projection objective for EUV lithography with a mirror sequence of concave-concave-convex-concave-convex-concave is shown in WO 99/57606. This objective has a numerical aperture at the object of NAobject=0.2. All mirrors of the system known from WO 99/57606 are made aspherical.
It is a disadvantage of the 6-mirror objective known from WO 99/57606 that there is no easy accessibility to the off-axis segments, for example, for mounting, particularly the second and third mirrors. In addition, the off-axis segment of the fourth mirror is arranged far outside the optical axis in the case of the system known from WO 99/57606. This leads to problems with respect to the stability of the mirror systems and in the manufacture of this mirror segment. Also, a large space is necessary in order to encapsulate the system. Since the system is utilized in vacuum, a relatively large space has then to be evacuated. The diaphragm arranged between the second and third mirrors according to WO 99/557606 has large angles of incidence on the third mirror as a consequence, which are particularly greater than 18°.
A 6-mirror system is known from U.S. 6,033,079, in which the angles of incidence on all mirrors are less than 18°. Of course, this system also has the disadvantage that the off-axis segment of the third mirror is not accessible and the off-axis segments of individual mirrors, for example, of the fourth mirror (M4), are so large that a large space to encapsulate the system is required, as in the case of the system known from WO 99/57606, which again leads to a relatively large space that must be evacuated. Another disadvantage of relatively large mirrors is their lack of stability and the fact that correspondingly large coating chambers and production equipment are required for their manufacture.